This invention relates to emulsion polymer based wood coatings and more particularly relates to a functional low Tg emulsion polymer in combination with a glycoluril derivative and a non-functional high Tg emulsion polymer to form a clear topcoat for plywood and hardboard interior paneling.
Clear topcoats for interior wood paneling have some rather stringent performance requirements which are extremely difficult to achieve in waterborne systems. These performance requirements include: clarity (no haziness or milkiness); block resistance (approximately 1 min. after topcoat is applied, panels are stacked face to face for shipping, and there can be no sticking of coated panels which are exposed to stack pressures as high as 120 psi); low temperature cure (clear topcoats must be capable of forming a film and curing in 20-30 sec. at F.B.S.T.'s of 165.degree.-205.degree. F.); variable gloss requirements are necessary for each wood species (therefore, the topcoat composition must provide some means for varying gloss); water resistance (the topcoat must not whiten or blister when exposed to high humidity or come directly in contact with water); tape release masking tape (e.g. Scotch brand #250 must be released by the substrate without damage to the finish); and stain resistance (the topcoat must not be damaged by several common household substances, e.g. detergent solutions, ammonia, solvents, lipstick, grape juice, nail polish remover, etc.).
Current coating compositions for wood coatings, particularly for interior wood paneling, are primarily solvent based wood coatings. Latex polymers are desirable waterborne binder systems for wood coatings, but are inadequate due to various characteristics of latexes. Block resistance is the most severe problem since the latex polymer must be soft enough to provide good cohesive film properties and yet be hard enough to resist sticking together of adjacent panels stacked one on top of the other for shipping or storage purposes. Since it is difficult to adjust the Tg of a latex polymer to obtain the proper balance of properties, the necessary filming properties are generally achieved at the expense of block resistance, and a product with unacceptable blocking properties is produced. It is known that a waterborne emulsion based clear coating with the proper balance of filming properties and block resistance is obtainable if the latex polymer composition contains a functional monomer that promotes crosslinking with an aminoplast resin. However, it has been our experience that a clear coating of this nature will not have the proper balance of properties at lauan plywood bake conditions unless the latex Tg is low enough to provide good coalescence at 165.degree.-205.degree. F., and the volume solids ratio of the crosslinking resin is greater than or equal to that of the latex polymer.
Even though the aforementioned coating composition has the desired balance of properties, it has not gained total acceptance by the paneling producers because of excessive costs and formaldehyde emissions. Since both of these coating parameters are directly proportional to the amount of aminoplast resin in the formulation, it becomes obvious that good filming properties and block resistance must be achieved at significantly reduced aminoplast resin content in order to satisfy all the desires of the paneling producers. As far as formaldehyde emissions are concerned, glycoluril crosslinkers are favored over urea or melamine formaldehyde types since they tend to generate significantly reduced levels of formaldehyde during the cure cycle.
It now has been found that highly desirable wood coatings can be produced based on a polymeric composition comprising a non-film forming high Tg emulsion polymer, sometimes referred to as plastic pigment, in combination with glycoluril and a low Tg emulsion polymer. The preferred composition comprises a reactive, low Tg polymer having reactive hydroxyl, carboxyl, or acrylamide groups adapted to be coreactive with glycoluril at relatively low temperatures in conjunction with a high Tg polymer, especially plastic pigment emulsions. Good cohesive film properties are achieved while maintaining superior block resistance comparable to commercial solvent based topcoats. These and other advantages of this invention will become more apparent by referring to the detailed description and illustrative examples.
U.S. Pat. Nos. 4,265,969 and 4,301,210 teach overcoating pigment with plastic pigment to improve surface gloss in coated paper.
Japanese Pat. No. JP 51/111277 (761001) teaches heat-sealing paper sheets by coating with a suspension of fine thermoplastic resin particles. Japanese Pat. No. JP 58/060091 (830409) teaches a pigment composition for coated paper where various pigments, including plastic pigment, is used with dextrose and emulsion copolymerized latex. Japanese Pat. No. 58/22974 (830721) teaches white ink composition for jet printing containing water-soluble resin, plastic pigment and water-soluble solvent. Japanese Pat. No. 58/054096 teaches a pigment composition (including plastic pigment) for coated paper in combination with an emulsion polymer latex. Japanese Pat. No. JP 58/046198 teaches an undercoating comprising various pigments including plastic pigment. Japanese Pat. No. JP 57/61193 (OJI) teaches paper coated with high solids (pigment dispersed in latex). U.S. Pat. Nos. 4,277,385 (810,707) and 4,283,320 (810,811) teach crack-free paints using an aqueous latex dispersion paint comprising 15-25 volume percent (dry solids basis) film-forming acrylic latex binders, 72-77% solid non-cellular, non-film forming polymer particles, and 3-8% opacified pigment of refractive index at 1.8. The binder has an average particle size of 0.1-0.5 microns and a Tg of at least 5.degree. C. below the coalescent temperature. The polymer particles have an average size 0.05-0.8 microns and have Tg at least 30.degree. C. above the Tg of the binder. None of the above patents teach how to develop block resistant thermoset clear coatings utilizing glycoluril crosslinking with plastic pigments.
Coassigned U.S. Pat. Nos. 4,283,320; 4,277,385; and 4,069,186, which relate generally to opaque coatings incorporating a variety of plastic pigments, are incorporated herein.